PF1A upgrade physics review Presented by D. A. Gates With input from J.E. Menard and C.E. Kessel 10/27/04
Outline History - how did we get here? –What were the limitations of the old PF1A? Physics optimization for target equilibrium Do we give anything up with the change? –and if so, what? –Survey of equilibria available with the new coils Summary
PF1A upgrade product of 5 yr. plan process Goal was to find an MHD stable 100% non-inductively sustained scenario w/ f bs ~ 70% at 40% t Unable to find scenarios/equilibria that satisfied requirements with original coil set (Kessel/Menard) Expectation was a requirement of simultaneous high and
High , not compatible with old PF1A For ~ 2.4, as is increased, outer squareness decreases For ~ 0.8, as is increased, second x- point forms Not conducive to MHD stability –Also not easily realized equilibria Problem is solenoid like shape of PF1A –Lots of field on the ends of the coil, little on the sides scan at high scan at high
Proposal to modify PF1A Based on observation that PF1A capable of making high at lower when plasma height is near the bottom of the PF1A coil Original proposal was to make 2 coils to maintain low high capability –No room for leads –proposed shifting present coil away from midplane
Final coil design Single ~half-height coil design located near the upper half of the original PF1A > Half as many turns (20 vs. 48) but more current (24kA vs 15kA) gives 2/3 amp-turn rating –Does not appear to limit operation (more later) New PF1A Old PF1A
PF1A upgrade required for T =40%, f NI = 100% Need N > 8 at > 2.4 for f BS =50-70% –Assumes profiles like –Remaining CD from NBI and EBW High- wall-stabilizes high- – limits drop 10-20% without high- (plates + vessel) Ideal-wall limit No-wall limit Ideal-wall limit Stability limits for high- + high- J. Menard
Steady State Scenario TSC used to demonstrate full time dependent non-inductively sustained scenario, consistent with transport and MHD stability and available external current drive (assuming EBW available) Time histories of the plasma density, temperature current for the fully non-inductive high scenario with TSC C. Kessel 5 yr. plan
Profiles indicate future challenges Profiles of parallel current, loop voltage, temperatures, density, safety factor and thermal diffusivities for fully non-inductive high scenario with TSC Requires shape control for high Also requires density control for non- inductive current C. Kessel 5 yr. plan
What does PF1A change imply for shape flexibility? scan gives much better shapes at high Squareness increases with increasing unlike with previous PF1A coil PF1A upgrade memo - D. Gates scan - Old PF1A scan - New PF1A
Equilibrium behavior As is scanned, q(95) increases by 50% with new PF1A relative to the old PF1A scan –Increased MHD stability Plasma parameters for scan –I p = 1.0MA –B t = 3kGauss – N = 6.0 Old PF1A New PF1A
Low high inaccessible with new PF1A Direct consequence of upgrade Could be repaired if deemed necessary by addition of second coil (lower half of old PF1A) PF1A current does not limit shape - hits PF2 limit first scan at fixed A
Coil currents from scan PF2 is limiting coil (for this scan)
Why now? Reduction in control system latency increased achievable –No further technical barrier to shape control for 5 year plan target equilibrium Center stack was removed early for additional TF repair Modification moved up to take advantage of opportunity Vertical stability diagram showing improved operating space for NSTX in
Benefits of increased confirmed Simultaneous doubling of t (pulse averaged) and 50 % increase in normalized pulse length Increase correlates strongly with high
Summary PF1A upgrade enables predicted 100% non- inductive operation with f bs ~ 70% and t ~40% –Indicates n=1 with wall stability for N > 9 –No wall limit N ~ 6 –(Also requires EBW and density control) Plasma vertical position control improvements have removed the major technical barrier to achieving this goal Reduction in operating space is tolerable –Give up low , high regime